Simplify SQL error handling (#2415)

* Remove sql::error submodule

Use anyhow errors instead.

* Remove explicit checks for open SQL connection

An error will be thrown anyway during attempt to execute query.

* Don't use `with_conn()` and remove it

* Remove unused `with_conn_async`

* Resultify markseen_msgs

src/config.rs

@@ -305,7 +305,7 @@ impl Context {
         }
     }
 
-    pub async fn set_config_bool(&self, key: Config, value: bool) -> crate::sql::Result<()> {
+    pub async fn set_config_bool(&self, key: Config, value: bool) -> Result<()> {
         self.set_config(key, if value { Some("1") } else { None })
             .await?;
         Ok(())